Physicists Play Ring-Around-the-Atom

NORMAL, Ill. -- Electrons racing about their atomic nuclei at nearly the speed of light bring new perspectives to the development of x-ray lasers. High-power, short-pulse lasers can coax electrons to yield x-rays, but they excite the electrons so much that the atoms can no longer hold onto them. A team at Illinois State University's Intense Laser Physics Theory Unit, however, has theorized that a strong magnetic field and a laser orders of magnitude less intense can produce the same effects in a more stable system.

The theory suggests that there is an alternative to the study of relativistic atomic electrons, but one that requires a large, static magnetic field, said Rainer Grobe and Qichang "Charles" Su, directors of the unit. They and their fellow researchers simulated the interaction of pulses from a 100-TW/cm2 CO2 laser and atomic electrons in a 1-kT magnetic field aligned to that of the laser. They predict in their report, which appeared in the April 10 issue of Physical Review Letters, that such a system could impel the electrons to speeds approaching 80 percent of the speed of light after a few cycles. It also could increase harmonic generation.

A theoretical model developed by researchers predicts that the stimulation of atomic electrons in a powerful magnetic field by a CO2 laser will accelerate them to relativistic speeds, forming the ringlike structure seen here, which they call a cycloatom. Courtesy of Illinois State University, Intense Laser Physics Theory Unit.
Particle accelerators can coerce electrons into relativistic conditions, but they carry a high price tag and need space. Tabletop lasers with intensities on the order of petawatts per square centimeter have yielded soft x-rays. They also are very expensive, if less so than the traditional accelerator, and the unwanted ionization effects of these lasers limit their usefulness.

The electrons in Grobe and Su's model do not break away from the atomic nuclei because the magnetic field produces an additional binding force that keeps them in their orbits. The electrons do display unexpected behavior, including the formation of structures that the researchers have dubbed "cycloatoms."

As the laser excites electrons in the field, they coalesce into a crescent-shaped charge cloud that grows to encircle the nucleus. The rings have an average kinetic energy of several hundred thousand electron volts. The researchers suggest that the collision of these cycloatoms in a high-density gas could produce novel energy bursts.
Although Grobe acknowledged that there could be challenges in developing a strong field, both researchers said that the work opens doors and that they are excited by the prospect of producing real cycloatoms.

"The dynamics of relativistic atomic electrons are not very well understood," said Su. "Our initial studies leading to the discovery of cycloatoms are just a first step in investigating the effects of relativity." The interaction of cycloatoms could shed light on high-order harmonic generation, antimatter and astrophysical processes.